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Next: <a href="Target-Macros.html#Target-Macros" accesskey="n" rel="next">Target Macros</a>, Previous: <a href="Loop-Analysis-and-Representation.html#Loop-Analysis-and-Representation" accesskey="p" rel="prev">Loop Analysis and Representation</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Machine-Descriptions"></a>
<h2 class="chapter">17 Machine Descriptions</h2>
<a name="index-machine-descriptions"></a>

<p>A machine description has two parts: a file of instruction patterns
(<samp>.md</samp> file) and a C header file of macro definitions.
</p>
<p>The <samp>.md</samp> file for a target machine contains a pattern for each
instruction that the target machine supports (or at least each instruction
that is worth telling the compiler about).  It may also contain comments.
A semicolon causes the rest of the line to be a comment, unless the semicolon
is inside a quoted string.
</p>
<p>See the next chapter for information on the C header file.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">&bull; <a href="Overview.html#Overview" accesskey="1">Overview</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">How the machine description is used.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Patterns.html#Patterns" accesskey="2">Patterns</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">How to write instruction patterns.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Example.html#Example" accesskey="3">Example</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">An explained example of a <code>define_insn</code> pattern.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="RTL-Template.html#RTL-Template" accesskey="4">RTL Template</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">The RTL template defines what insns match a pattern.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Output-Template.html#Output-Template" accesskey="5">Output Template</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">The output template says how to make assembler code
                        from such an insn.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Output-Statement.html#Output-Statement" accesskey="6">Output Statement</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">For more generality, write C code to output
                        the assembler code.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Predicates.html#Predicates" accesskey="7">Predicates</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Controlling what kinds of operands can be used
                        for an insn.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Constraints.html#Constraints" accesskey="8">Constraints</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Fine-tuning operand selection.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Standard-Names.html#Standard-Names" accesskey="9">Standard Names</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Names mark patterns to use for code generation.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Pattern-Ordering.html#Pattern-Ordering">Pattern Ordering</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">When the order of patterns makes a difference.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Dependent-Patterns.html#Dependent-Patterns">Dependent Patterns</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Having one pattern may make you need another.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Jump-Patterns.html#Jump-Patterns">Jump Patterns</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Special considerations for patterns for jump insns.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Looping-Patterns.html#Looping-Patterns">Looping Patterns</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">How to define patterns for special looping insns.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Insn-Canonicalizations.html#Insn-Canonicalizations">Insn Canonicalizations</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Canonicalization of Instructions
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Expander-Definitions.html#Expander-Definitions">Expander Definitions</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Generating a sequence of several RTL insns
                        for a standard operation.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Insn-Splitting.html#Insn-Splitting">Insn Splitting</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Splitting Instructions into Multiple Instructions.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Including-Patterns.html#Including-Patterns">Including Patterns</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Including Patterns in Machine Descriptions.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Peephole-Definitions.html#Peephole-Definitions">Peephole Definitions</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Defining machine-specific peephole optimizations.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Insn-Attributes.html#Insn-Attributes">Insn Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Specifying the value of attributes for generated insns.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Conditional-Execution.html#Conditional-Execution">Conditional Execution</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Generating <code>define_insn</code> patterns for
                         predication.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Define-Subst.html#Define-Subst">Define Subst</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Generating <code>define_insn</code> and <code>define_expand</code>
			patterns from other patterns.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Constant-Definitions.html#Constant-Definitions">Constant Definitions</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Defining symbolic constants that can be used in the
                        md file.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Iterators.html#Iterators">Iterators</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Using iterators to generate patterns from a template.
</td></tr>
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